Currently the major process by which hydrogen is produced is by the steam reforming of methane. Another means of making hydrogen is by the electrolysis of water. The electricity required for electrolysis is mainly derived from the electric power grid, and the predominant source of grid electricity, combustion of fossil fuels, generates emissions, such as nitrogen oxides and particulate matter, as well as carbon dioxide. One way to eliminate such emissions is to use solar generated electricity to electrolyze water to make hydrogen. Presently, efforts are directed toward improving the efficiency, durability, and cost of the solar powered hydrogen production processes.
Photovoltaic (PV) or solar cells can be used to supply the electricity necessary for the electrolysis of water. A single solar cell is the smallest unit of a photovoltaic system and usually has to little voltage to drive electrolysis. A group of solar cells called a photovoltaic module are electrically connected together in series and/or in parallel to supply sufficient voltage and current for various uses. Modules are normally encapsulated in a transparent, waterproof structure. However, systems consisting of solar cells in PV modules to make electricity together with electrolyzers to dissociate water into hydrogen and oxygen, as they exist today, cannot produce hydrogen as cheaply as the steam reforming of methane. Several projects have attempted to produce hydrogen gas to supply vehicle-fueling stations by using electricity from photovoltaic modules and commercially available electrolyzers to split water. These projects proved unsatisfactory due to the low efficiency and high cost of the combined technology, which only converted about 2%-6% of the solar energy to hydrogen fuel energy, thus, greatly increasing capital costs, the resulting hydrogen fuel cost (at least $15 per kilogram of hydrogen), and the large area covered by the system. That technology was based on non-optimal PV and electrolyzer combinations, and involved large land areas for collecting sufficient solar energy.
Other methods for converting solar energy into hydrogen are disclosed in co-assigned U.S. Provisional Application 60/545,379, filed on Feb. 18, 2004; and application U.S. Ser. No. 11/049,213, filed on Feb. 2, 2005, entitled “Method and Apparatus for Hydrogen Generation” and in Provisional Patent Application U.S. Ser. No. 60/545,374, filed on Feb. 18, 2004, and application U.S. Ser. No. 11/046,572, filed on Jan. 28, 2005, entitled “Hydrogen Generator Photovoltaic Electrolysis Reactor System”, which are incorporated herein by reference. These apparatuses describe the optimization of the solar-cell powered electrolysis of water to produce hydrogen using electrolysis cells in reactors containing a liquid alkaline electrolyte. Each of those cells produces about one gram (12 L at STP) of hydrogen per day. It was found that a specific optimum operating voltage for an electrolyzer cell using Ni and Ni—RuO2 plates for the cathode and anode respectively, and with a 5 M KOH electrolyte solution existed which was approximately 2.0-2.5 volts which must be matched by the maximum power point voltage, (Vmpp) of the PV system.] [Note: Whenever, the electrical units, volts, amps, etc. are used anywhere in this document, the electrical units are of direct current (DC) and not alternating current (AC).] While a significant improvement over previous systems, these small tank or bag reactors are relatively bulky and would need to be scaled up by several hundred-fold to fuel one fuel cell vehicle.